POLYACETAL COMPOSITION

Abstract: To provide a type of polyacetal composition appropriate for manufacturing sliding parts with excellent friction/wear characteristics. A type of polyacetal composition characterized by the fact that the composition contains 0.5-15 wt% (with respect to the total weight of the composition) of a polypropylene glycol elastomer crosslinked by sulfur as a copolymer made of 90-99.5 wt% of propylene glycol and 0.5-10 wt% of allyl glycidyl ether, or, the composition contains 0,5-10 wt% (with respect to the total weight of the composition) of a polypropylene glycol elastomer crosslinked by sulfur as a copolymer made of 90-99.5 wt% of propylene glycol and 0.5-10 wt% of allyl glycidyl ether, along with 0.5-10 wt% (with respect to the total weight of the composition) of polyalkylene glycol oil.

Description: TITLE POLYACETAL COMPOSITION DETAILED EXPLANATION OF THE INVENTION This invention concerns a type of resin composition appropriate for manufacturing sliding parts with excellent friction/wear characteristics. Usually, polyacetal resins have better self-lubricating properties than metals. When two parts made of the same type of polyacetal resin are brought in contact with each other, the frictional coefficient is small and the wear resistance is high. Consequently, the polyacetal resin is very useful in manufacturing bearings, gears, rollers, cams, guide rails, and other parts with low noise levels in operations. In order to improve the friction/wear characteristics of the polyacetal resin, in the conventional scheme, polypropylene glycol, polyethylene glycol, polytetramethylene glycol, propylene glycol-ethylene glycol copolymer, or some other polyalkylene glycol is added. However, when a small amount of the polyalkylene glycol is added, the improvement in the friction/wear characteristics is insignificant. On the other hand, when a large amount of the polyalkylene glycol is added, when the screw of the an injection molding machine or extrusion molding machine is moved backwards, the pellets slide on the inner surface of the cylinder and are caught in the molding machine, significantly reducing the productivity, which is a disadvantage. On the other hand, when a polypropylene glycol elastomer is added to the polyacetal resin, although the impact resistance is improved, there is nevertheless no significant effect with respect to improvement of the friction/wear characteristics (U.S. Patent No.3,631,124). It is believed that this is caused by the fact that polypropylene glycol is a straight-chained elastomer, and that the amount of glycidyl ether used as the crosslinking agent is small. Consequently, there is a demand for the development of a type of polyacetal resin composition free of the phenomenon of slipping into the screw, which used to take place for the conventional polyacetal composition for sliding parts by adding a polypropylene glycol elastomer to the polyacetal resin. In order to meet the aforementioned demand, the present inventors have performed extensive research. As a result of this research work, it was found that the composition prepared using sulfur to crosslink a copolymer made of propylene glycol and allyl glycidyl ether to the polyacetal resin, or prepared by adding the aforementioned copolymer and polypropylene glycol oil to the polyacetal resin, has excellent friction/wear characteristics, and with excellent producibility for injection molding or extrusion molding. That is, this invention provides a type of polyacetal composition characterized by the fact that the composition contains 0.5-15 wt (with respect to total weight of the composition) of a polypropylene glycol elastomer crosslinked by sulfur as a copolymer made of 90-99.5 wt% of propylene glycol and 0.5-10 wt% of allyl glycidyl ether. Also, this invention provides a type of polyacetal composition characterized by the fact that the composition contains 0.5-10 wt% (with respect to the total weight of the composition) of a polypropylene glycol elastomer crosslinked by sulfur as a copolymer made of 90-99.5 wt% of propylene glycol and 0.5-10 wt% of allyl glycidyl ether, along with 0.5-10 wt% (with respect to the total weight of the composition) of polyalkylene glycol oil. The polyacetal resin used in this invention is of the conventional type. It may be formed by the polymerization of formaldehyde, trioxane, tetraoxane, etc., as cyclic oligomers of formaldehyde, etc. It may also be formed by the copolymerization of the aforementioned compounds with a small amount of acetaldehyde, propionaldehyde, or aldehyde other than formaldehyde or a cyclic oligomer of formaldehyde. It can also be formed by the copolymerization of these aldehydes and a cyclic ether or cyclic acetal, such as ethylene oxide, propylene oxide, 1,3-dioxolane, etc. The polyacetal resin may have a main chain made of — (CH 2 0) n — units (where n is a natural number) and/or — (CHR— 0) n — units (where R represents an alkyl group and n represents a natural number), with unprotected terminals. It may also be a linear polymer protected by — OCOCH — , — OCH3— , — OCH 2 — OH, or other groups, with a number-average molecular weight of 10,000-100,000, preferably 20,000-70,000. However, since the addition of the lubricant can lead to degradation of the physical characteristics (such as tensile characteristics, Izod impact characteristics, bending characteristics, compression characteristics, etc.) for some types of copolymers, it is preferred to use the homopolymer form of the polyacetal. The amount of the polypropylene glycol elastomer added to the polyacetal composition in this invention is made of a copolymer consisting of 90-99.5 wt of propylene glycol and 0.5-10 wt% of allyl glycidyl ether, crosslinked by sulfur. The polypropylene glycol elastomer is a type of polyether having allyl groups formed in the copolymerization of propylene oxide and allyl glycidyl ether via its side chains, and crosslinked by sulfur. In the polyacetal composition, the amount of polypropylene glycol elastomer added with respect to the total weight of the polyacetal resin should be 0.5-15 wt%. If the amount added is less than 0.5 wt%, there is no effect in improving the friction/wear characteristics. On the other hand, if the amount is larger than 15 wt%, the intrinsic properties of the polyacetal resin are degraded, and the processability is poor. The preferably amount is 1-10 wt%, or more preferably 2-5 wt%. As a form for implementing this invention, a polyalkylene glycol oil is added to the polyacetal composition in addition to the polypropylene glycol elastomer. In this case, the amount of the polypropylene glycol elastomer in the polyacetal composition should be 0.5-10 wt%. The amount of the polyalkylene glycol oil in the polyacetal resin composition should be 0.5-10 wt%. According to this invention, the polyalkylene glycol oil used is an oil-like polymer, which can be formed by the polymerization or copolymerization of an alkylene oxide, such as ethylene oxide, propylene oxide, tetrahydrofuran, etc., with a polymerization degree of 5-1000. Examples of the polyalkylene glycol oil that can be used include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, propylene glycol-ethylene glycol copolymer, etc. Among these, polypropylene glycol is preferred. The amount of the polyalkylene glycol oil with respect to the weight of the polyacetal composition is 0.5-10 wt%. If the amount is smaller than 0.5 wt%, there is little effect in improving the friction/wear characteristics. On the other hand, if the amount is larger than 10 wt%, the properties of the composition are degraded. This is undesirable. The preferably range of the content is 1-5 wt%. A conventional method can be used for preparing the polyacetal resin composition of this invention. For example, any of the following methods may be used: method in which the various components are blended, followed by extrusion from an extrusion molding machine, or the various components are respectively added to the extrusion molding machine for blending; method in which the various components are blended by a kneader or other equipment; method in which the various components are blended by a kneader or other equipment; method in which the various components are dissolved in an appropriate solvent, or they are respectively dissolved in the solvent, followed by blending, then removal of the solvent; or a mixture of these methods. Various additives for improving the properties of the polyacetal resin may be used with the polyacetal resin composition of this invention. Examples of the additives that can be used include a thermal stabilizer for improving the thermal stability in molding, UV absorbent for improving the weatherability, optical stabilizing agent, nuclear agent, static inhibitor, various coloring agents, pigments, etc. They may be added in amounts that do not harm the characteristics of the polyacetal resin, also, it is possible to add glass fibers, mineral, talc, or other fillers. For the [conventional] polypropylene glycol elastomer, since polypropylene glycol is a straight-chained polymer, and the amount of glycidyl ether added as the crosslinking agent is small, the friction/wear characteristics of the polyacetal resin cannot be significantly improved. However, according to this invention, instead of the conventional straight- chained elastomer, the type of polypropylene glycol elastomer used is prepared by the copolymerization of propylene glycol and allyl glydicyl ether and with crosslinking by sulfur. Consequently, it has a higher crosslinking degree than that of the conventional polypropylene glycol elastomer, and hence the friction/wear characteristics of the polyacetal resin can be significantly improved. In addition, since both the polypropylene glycol elastomer and polyalkylene glycol oil are added to the polyacetal resin, the friction/wear characteristics can be improved by the synergic effect. In the following, this invention will be explained in more detail with reference to application examples and comparative examples. However, this invention is not limited to these examples. APPLICATION EXAMPLES 1-4 With the proportions listed in Table I in the following, polyacetal resin (Delrin ® 500 NC, a type of polyacetal homopolymer produced by the DuPont Co.), polypropylene glycol elastomer (Pearl ® , product of the [Nippon] Zeon Co.), and polyalkylene glycol oil (PPG, product of the Dow Corning Co.) were melted and blended in a biaxial extruder (Toshiba TEM35), followed by water cooling to manufacture pellets. In order to perform the tests for the friction/wear characteristics, the resin pellets were molded into sheet-shaped specimens using an injection molding machine. The tests for the friction/wear characteristics of the specimens were performed as follows. Test Methods for Friction/Wear Characteristics The polyacetal resin composition sheet (specimen 1) (test piece) was set on a table plate that allows reciprocating movement in the horizontal direction with a width of 2.75 in. On the other hand, a plate (specimen 2) made of a polyacetal resin (Delrin ® 500 NC, product of the DuPont Co.) with a width of 1/4 in. and a length of 1 in. as the other part of the sliding movement was fixed without undergoing the horizontal movement, and in contact with the aforementioned polyacetal resin plate. A load of 40 psi was applied on sample 2, then the table plate was set in a reciprocating movement at a mean velocity of 30 ft/min for 3 h. The loss in weight after the test was measured for each of these two resin plates, then the friction/wear characteristics were calculated using the following formula: W = K x F x V x T W = wear volume (in 3 ) K = frictional coefficient (in 3 -min/ft-lb-h) F = force (lb) V velocity (ft/min) T = time (h) W = (weight loss of specimen 1 + weight loss of specimen 2) / (mean density of specimens 1 and 2). The results are listed in Table I. Comparative Examples 1-3 The test for the friction/wear characteristics was performed in the same way as for Application Examples 1-4, except that either only the polyacetal resin (Delrin ® 500 NC, product of the DuPont Co.) was used for specimen 1 (in Comparative Example 1), or the polyacetal resin (Delrin® 500 NC, product of the DuPont Co.) and polypropylene glycol oil (PPG- 1000, product of the Dow Corning Co.) were used for specimen 1 (in Comparative Example 2), or the polyacetal resin (Delrin ® 500 NC, product of DuPont Co.) and a polyurethane elastomer (Texin 480, product of the Bayer Co.) were used for specimen 1 (in Comparative Example 3), and the same specimen was used for specimen 2 as in the application examples. The obtained results are also listed in Table I. Table I Component Application Application Application Application Comparative Comparative Comparative (wt%) Example 1 Example 2 Example 3 Example 4 Example 1 Example 2 Example 3 Polyacetal 97.5 90.0 98.0 95.5 100 98.0 90.0 PPG Elastomer 2.5 10.0 1.1 2.5 0 0 0 PPG Oil 0 0 0.9 2.0 0 2.0 0 Polyurethane Elastomer 0 0 0 0 0 0 10.0 Frictional Coefficient 9000 4000 7000 4000 10000 44000 8000 As can be seen from Table I, the frictional coefficients of the compositions in Application Examples 1-4 are smaller than those in Comparative Examples 1-2. When the composition of Application Example 2 is compared with the composition of Comparative Example 3, it can be seen that when the same amount of elastomer is added, the composition of Application Example 2 has better friction/wear characteristics. When the compositions of Application Examples 1 and 3 are compared with each other, and when the compositions of Application Examples 2 and 4 are compared with each other, it can be seen that when both polypropylene glycol elastomer and polypropylene glycol oil are added, there is a little improvement for the friction/wear characteristics.

Claims: CLAIMS 1. A polyacetal composition containing 0.5-15% based on the weight of the composition, of a sulfur-crosslinked polypropylene glycol elastomer, comprising a copolymer of 90-99.5% of propylene glycol and 0.5- 10% by weight of allyl glycidyl ether. 2. A polyacetal composition containing 0.5-10% based on the weight of the composition, of a sulfur-crosslinked polypropylene glycol elastomer, comprising a copolymer of 90-99.5% by weight of propylene glycol and 0.5-10% by weight of allyl glydicyl ether and 0.5-10% by weight based on the weight of the composition of a polyalkylene glycol oil.

Inventors: SHINOHARA KENICHI, TAKAHASHI TATSUHIRO, SHINOHARA, KENICHI, TAKAHASHI, TATSUHIRO
Applicants: DU PONT, SHINOHARA KENICHI, TAKAHASHI TATSUHIRO, E.I. DU PONT DE NEMOURS AND COMPANY, SHINOHARA, KENICHI, TAKAHASHI, TATSUHIRO
Application year: 1994
Main classification: C08L71
IPC codes: C08G65, C08L59, C08L71, F16D69